This invention relates to pulse circuits which are capable of generating high power pulses of short duration. A circuit of this kind can be used to provide the operating power for a high power oscillator, such as a magnetron, which forms part of a radar transmitter. Such a pulse circuit is sometimes termed a radar pulse modulator. A radar transmitter transmits pulses having a very low mark-to-space ratio, that is to say, transmitted short pulses are spaced apart in time by relatively long intervals during which echoes of the pulses are returned by intercepted radar targets to a radar receiver. The useful range of a radar is related to the power transmitted during the short pulse periods and it is therefore very important to maximimise the power of these pulses, whilst ensuring that the pulses turn on and turn off cleanly without the generation of excessive noise. Following the turn off, or decay, of a transmitted short pulse, the receiver of the radar is enabled so that it can detect weak radar echoes. It is clearly important to ensure that the trailing edges of the transmitted short pulses decay very rapidly and cleanly so that they do not mask echoes received after only a very short delay from targets at very close range.
It is customary to use pulse forming networks to generate pulses of the required characterstics. In a pulse forming network, an array of capacitors and one or more inductors are charged to a required voltage level, and when fully charged the network is discharged, typically into a pulse transformer which raises the voltage to a level (typically 30 kV) at which it can be used to drive a magnetron oscillator. It has previously been usual to use a hydrogen thyratron or a similar device, as a switch to discharge the pulse forming network. Because thyatrons can operate at reasonably high voltages (e.g. several thousand volts), the current handling capacity of such pulse forming networks has been relatively modest.
Although in principle it is becoming desirable to use switchable semiconductor devices to discharge a pulse forming network, severe difficulties are encountered in practice in designing and manufacturing a network which is capable of handling with sufficient precision and consistency the very large currents involved. Because semiconductor switches are not capable of handling relatively large voltages, the pulse current must be proportionately higher to enable the overall power of the system to be sufficiently great, and the present invention seeks to provide an improved pulse circuit and a pulse forming network which is suitable for use in a pulse circuit which uses solid state switches.